There have been proposed many processes which produce porous particles or hollow particles, for example, (i) W/O/W suspension polymerization (see Japanese Kokai Publication (unexamined) 72003/1986), (ii) a method wherein a monomer mixture is mixed with an organic solvent and suspension-polymerized, followed by expanding the solvent out, and (iii) a method wherein two kinds of polymerization initiators which have different reaction temperature with each other are employed and polymerized.
However, in the process (i), a large amount of a surfactant remains in obtained particles, but it is very difficult to remove the surfactant from the particles. In the process (ii), the organic solvent which is unnecessary for a water based system remains and many efforts are required to remove it. In the process (iii), a particular reaction vessel is required, because the temperature of the second reaction reaches to approximate 150 .degree. C.
In view of encapsulization technique, capsules having inorganic walls and their producing methods have also been proposed, but the capsules have poor affinity with organic materials and therefore are difficult to mix with the organic materials. Also, since the walls are constituted through ion bonding, the water resistance of the capsules is very poor.